Abstract: In order to study the surrounding pressure affected to the replacement effect in the process of CO2 injection to replace CH4 in coal seam,based on cylindric al raw coal specimen from Qinshui Coalfield as the study object,a experiment of CO2 injection to replace CH4 was conducted under different surrounding pressure conditio n.The results showed that in the process of CO2 injection replacement experiment,the CO2 adsorption capacity of coal was higher than CH4. The concentration of adsorbed phase CO2 in the coal would be reduced with the surrounding pressure increased,and increased with the injection pressure decreased and the concentration of adsorbed p hase CH4 would be in a reverse varied tendency.During the experiment,the CH4 desorption rate was higher than that of CO2 with the specific pressure decreasing.Meanwh ile with the surrounding pressure increased,the CH4 desorption rate at the specific pressure decrease would be reduced in sequence and would be a better replacement eff ect in low surrounding pressure period.Under the surrounding pressure condition of 8, 12,16 and 20 MPa,in CO2 injection replacement experiment,CH4 desorption rate with the specific pressure decreased was improved by 119.6%,112.7%,107.5% and 106 .5% individually relative to the pure CH4 desorption experiment,thus CO2 injection in the seam could clearly and effectively improve the CH4 recovery rate and the replacement effect under the condition of the low surrounding pressure would be more obvious.